The present invention relates, in general, to control systems; and, in particular, to circuitry for interconnecting a reference signal generator to a control system.
In a control system, a reference signal sets the desired performance of the controlled vehicle. The control system will follow the reference signal. If the reference signal becomes invalid, then the control system itself will become invalid. One type of control system is a closed loop control system for a prime mover, for example, a steam turbine. In a steam turbine control system, a reference signal to the control system is compared with a feedback signal from the turbine to generate an error signal which is then used to adjust the steam admission valves until the feedback signal matches the reference signal. The reference signal is provided by a reference signal generator which may, for example, be a computer. The output of the reference signal generator is dependent upon the input information it receives and the proper functioning of the reference signal generator itself. If the input information is invalid or the reference signal generator itself fails, then it is possible to have a catastrophic failure of the reference signal generator output. The foregoing mode of failure may not affect, too adversely, the controlled vehicle; but it is possible that such a failure will greatly affect, in an adverse manner, equipment which interfaces with the controlled vehicle. For example, a boiler feed pump turbine supplies feedwater to a boiler. The turbine itself includes overspeed protection and will not be too adversely affected by shutdown or overspeed trip. However, the boiler which interfaces with the turbine through a boiler feed pump may be subject to excessive or insufficient feedwater supply with damaging consequences.
It is clear, therefore, that one object of the present invention is to discontinue an invalid reference signal generator output to the control system.
If the reference signal generator output to the control system is discontinued, then an alternate reference signal to control system may be provided. This alternate signal can be provided by switching to a manually adjustable input reference signal. However, a problem arises in the selection of an appropriate set point for the manually adjustable reference signal which must be at or near the last valid output of the reference signal generator.
It is therefore another object of the present invention to provide an automatic alternate reference signal which is at the last valid output of the reference signal generator.
It is a combined objective of the present invention to provide a reference signal to a control system which will follow a reference signal generator output which is valid; and, which will provide an alternate last valid reference signal for an invalid reference generator output when the invalid reference generator output is detected.
It is a further objective of the present invention to hold the control system at the last valid reference signal indefinitely until the reference signal generator malfunction is corrected or until manual control may be asserted at the last valid input reference signal level.
While the foregoing has been illustratively described in terms of a closed loop control system for a prime mover, it is not intended to limit the application of the present invention thereto.
The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof, may best be understood with reference to the following description, taken in connection with the drawings.